1. Field of the Invention
The present invention generally relates to a micromechanical device for use within infrared imaging devices. Specifically, the invention is a micromechanical pixel including improved sensing and bending elements which separately and in combination increase the sensitivity and decrease the response time of the pixel.
2. Description of the Related Art
Infrared imaging devices enable a user to view an object via the infrared band of the spectrum, which is otherwise invisible to the human eye. Infrared imaging devices are applicable to security and surveillance, firefighting, automotive safety, and industrial monitoring because the peak thermal emission of objects in such applications is centered within the infrared region. However, the high cost of infrared imaging devices remains a challenge, thereby limiting their use.
The radiation detectors employed within imaging devices are either photon detectors or thermal detectors.
Photon detectors produce an image when incident radiation is absorbed within a sensing material via interactions with electrons bound to lattice or impurity atoms or with free electrons. An output signal, in the form of a voltage or current change, is produced by changes in the electronic energy distribution. The materials used in photon detectors, typically HgCdTe and InSb, exhibit very high quantum efficiency in the infrared band. However, photon detectors must be cryogenically cooled, thus increasing the weight, volume, and power consumption of presently known devices. Furthermore, materials which are highly quantum efficiency are notoriously difficult to process and costly. As such, imaging devices based on photon detector technologies are limited to specialized applications within the fields of national defense and astronomy.
Thermal detectors produce an image when incident radiation is absorbed by a thermally-sensitive material that alters some physical property of the material, examples including resistance or capacitance. The alteration of the physical property is typically detected by a readout integrated circuit (ROIC), which generates an output signal. Thermal detectors operate at room temperature, thus avoiding the cooling required by and complexity of photon detector devices; however, the performance of thermal detectors, as measured by their noise equivalent temperature difference (NETD), is approximately ten times less sensitive than photon detectors. The thermal sensitivity of detector materials, examples including vanadium oxide or amorphous silicon, is characteristically in the range of 2%/K to 3%/K. The bias of an interrogation pulse from a ROIC, which controls detector responsivity, is restricted to prevent self-heating of a pixel. While less costly than photon detector devices, thermal detector devices are affordable within the fields of industrial monitoring and firefighting, yet too costly for most consumer and many industrial applications.
Thermal imaging devices employing passive thermal bending, composed of bi-layer micro-cantilevers for temperature and radiation sensing and electrical, capacitive, or optical readout, are described within the related arts. For example, FIG. 1a shows an exemplary bi-layer cantilever 50 including a first layer 1 contacting and attached to a second layer 2 which are thereafter attached to a stationary support 51. The first layer 1 has a coefficient of thermal expansion different from that of the second layer 2. FIG. 1b shows the same bi-layer cantilever 50 after the first layer 1 and second layer 2 are heated by infrared radiation 52 causing the bi-layer cantilever 50 to bend. This approach to thermal imaging eliminates the monolithic integration of a pixel-level ROIC, further eliminating electronic noise and simplifying device fabrication. However, the sensitivity of presently known cantilevers is poor because of their low absorption efficiency and large mass.
An improved micromechanical thermal imaging device is described by Ishizuya et al. in U.S. Pat. Nos. 6,080,988, 6,339,219, 6,469,301, and 6,835,932. Referring now to FIGS. 2-4, a micromechanical pixel 3 is shown including a sensing element 4 disposed between and separated from a pair of bending elements 5a, 5b adjacent to a substrate 9. The sensing element 4 includes an optical absorption cavity 7 bounded by an absorber layer 8 and a reflector 6 which are spaced apart by and attached to a support post 29, as shown in FIG. 3. Each u-shaped bending element 5a, 5b is composed of a pair of bi-layer cantilevers 10a, 11a and 10b, 11b. Each paired arrangement of bi-layer cantilevers 10a, 11a and 10b, 11b is separated by a thermal isolation region 12a, 12b having a low thermal conductance. Each bi-layer cantilever 10a, 10b, 11a, 11b is composed of a high expansion layer 33 which contacts and is attached to a portion of a low expansion layer 34, as shown in FIG. 4, opposite of the substrate 9. The low expansion layer 34 of the innermost bi-layer cantilevers 11a, 11b is attached to the sides of the absorber layer 8, as represented in FIG. 2. Bending elements 5a, 5b are attached to the substrate 9 via a pair of anchor posts 13a, 13b so that a gap 49 is provided between the sensing element 4 and substrate 9 and between the bending elements 5a, 5b and substrate 9. The height of the gap 49 may be adjusted by making the length of the innermost bi-layer cantilevers 11a, 11b shorter than the outermost bi-layer cantilevers 10a, 10b. 
In the absence of infrared illumination, the outermost bi-layer cantilevers 10a, 10b negate the deflection of the innermost bi-layer cantilevers 11a, 11b, thus producing a net bending of zero so as to maintain zero tilt along the sensing element 4, regardless of the change in ambient temperature. When illuminated via an infrared source, the optical absorption cavity 7 receives and converts infrared radiation into heat which is conducted into the innermost bi-layer cantilevers 11a, 11b, resulting in additional bending with respect to the outermost bi-layer cantilevers 10a, 10b and causing the sensing element 4 to tilt with respect to the plane of the substrate 9. Proper function of the device in FIG. 2 requires the micromechanical pixel 3 to be backside illuminated 32, whereby infrared radiation is transmitted through the substrate 9. High sensitivity is achieved via an efficient, yet lightweight, sensing element 4 and thin bi-layer cantilevers 10a, 10b, 11a, 11b. However, the micromechanical pixel 3 in FIG. 2 suffers from several deficiencies, which limit sensitivity and contribute to sensor noise, including a low fill factor, poor reflector flatness, and mechanical complexity.
The micromechanical pixel 3 described in FIGS. 2-4 is applicable to a variety of detectors. For example, FIG. 5 shows an exemplary optical readout device 28 described by Ishizuya et al. in U.S. Pat. No. 6,339,219 which includes an infrared lens system 15, an infrared detection array 16, a first lens system 19, an aperture plate 22, a second lens system 24, and an imager 25 arranged in the order described. Within the front end of the apparatus, rays from a source 14 pass through the infrared lens system 15 and are thereafter directed onto the infrared detection array 16. The infrared detection array 16 includes a focal plane array 17 composed of micromechanical pixels 18 which are mechanically responsive to the thermal loading induced by the infrared rays. Within the back end of the apparatus, micromechanical pixels 18 reflect the incident light 20 from a visible light source 23, one example being a light emitting diode (LED), so that the reflected light 21 passes through the first lens system 19 which compresses the reflected light 21 allowing it to pass through the pinhole 53 along the aperture plate 22. The reflected light 21 then passes through the second lens system 24 which expands the reflected light 21 so as to impinge a focal plane array 27 composed of receptor pixels 26 within the imager 25, examples being a complementary metal oxide semiconductor (CMOS) device or charged-coupled device (CCD). Thereafter, the resultant image is communicated to a video display device.
The detector in FIG. 5 employs an optical system to simultaneously measure the deflections of all micromechanical pixels 18 so as to project a visible image of spatially-varying infrared radiation directly onto a commercial-off-the-shelf visible CMOS or CCD imager. The number of receptor pixels 26 within the CMOS or CCD array is generally chosen to be more than the number of micromechanical pixels 18. In operation, an image produced by the detector in FIG. 5 is of uniform intensity over the entire array of receptor pixels 26 when no illumination is present because of the canceling effect of the paired arrangement of bi-layer cantilevers 10a, 11a and 10b, 11b, as described above for FIGS. 2-4. When illuminated by an infrared source, a sensing element 4 tilts within each micromechanical pixel 18 and deflects light away from the pinhole 53, thus projecting darker receptor pixels 26 with intensities which are proportional to the radiation level. The detector effectively converts infrared radiation into intensity change at a visible or near-infrared readout wavelength.
The micromechanical pixel 3 in FIG. 2 produces design related noise including: (1) noise caused by the radiative heat exchange between each pixel and its environment, referred to as background fluctuations; (2) noise caused by the dynamic heat exchange between each pixel and the substrate, referred to as thermal fluctuations; (3) noise from mechanical energy stored in the cantilever continuously exchanged with thermal energy, referred to as thermomechanical noise; and (4) noise caused by the random arrival rate of photons at the CMOS/CCD imager, referred to as shot noise. Since all noise sources are probabilistic, the total NETD for a micromechanical IR imager is equal to the square root of the sum of the squares of the contributing noise sources and is given byNETDTOT=√{square root over (NETDBF2+NETDTF2+NETDTM2+NETDSN2)},  (1)where the subscripts BF, TF, TM, and SN refer to the NETD due to background fluctuations, thermal fluctuations, thermomechanical noise, and shot noise, respectively. The background fluctuation NETD is given by
                                          NETD            BF                    =                                                    2                ⁢                                  (                                                            4                      ⁢                                              f                        2                                                              +                    1                                    )                                                                              ɛτ                  0                                ⁢                η                ⁢                                                      ⅆ                    P                                    /                                      ⅆ                    T                                                                        ⁢                                                            2                  ⁢                                      k                    B                                    ⁢                  σ                  ⁢                                                                          ⁢                                      B                    ⁡                                          (                                                                        T                          D                          5                                                +                                                  T                          B                          5                                                                    )                                                                      A                                                    ,                            (        2        )            where f is the f-number of the lens, ε is the pixel emissivity, τ0 is the transmission of the optics, η is the pixel absorption efficiency, dP/dT is the differential irradiance, kB is Boltzmann's constant, σ is the Stefan-Boltzmann constant, B is the thermal bandwidth, TD is the detector temperature, TB is the background temperature, and A is the active pixel area.
The NETD due to thermal fluctuations is given by
                                          NETD            TF                    =                                    2              ⁢                              (                                                      4                    ⁢                                          f                      2                                                        +                  1                                )                            ⁢                              T                D                            ⁢                                                                    k                    B                                    ⁢                  BG                                                                                    τ                0                            ⁢              η              ⁢                                                          ⁢              A              ⁢                                                ⅆ                  P                                /                                  ⅆ                  T                                                                    ,                            (        3        )            where G is the thermal conductivity.
The NETD due to thermomechanical noise is equal to
                                          NETD            TM                    =                                                    2                ⁢                                  (                                                            4                      ⁢                                              f                        2                                                              +                    1                                    )                                ⁢                G                                                              ητ                  0                                ⁢                A                ⁢                                                                  ⁢                ℓ                ⁢                                                      ⅆ                    P                                    /                                      ⅆ                    T                                                  ⁢                                                                  ⁢                ℜ                                      ⁢                                                                                k                    B                                    ⁢                                      T                    D                                    ⁢                  B                                                  kQ                  ⁢                                                                          ⁢                                      ω                    0                                                                                      ,                            (        4        )            where l is the length of the bimaterial cantilever,  is the pixel responsivity (defined as the change in pixel deflection angle per degree Kelvin), k is the stiffness of the cantilever, Q is the cantilever Q-factor, and ω0 is the cantilever resonant frequency.
The NETD due to shot noise is given by the expression
                                          NETD            SN                    =                                                                      (                                                            4                      ⁢                                              f                        2                                                              +                    1                                    )                                ⁢                G                                                              ητ                  0                                ⁢                A                ⁢                                                      ⅆ                    P                                    /                                      ⅆ                    T                                                  ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                P                                      ⁢                                                            2                  ⁢                  qPB                                                  ℜ                  c                                                                    ,                            (        5        )            where P is the visible light power received by a CMOS/CCD pixel, ΔP is the change in light power per degree Kelvin (where ΔP∝), q is the elementary charge, and c is the responsivity of the CMOS/CCD imager.
The dominant source contributing to the NETD in a micromechanical pixel is typically the shot noise NETD. The shot noise NETD may be lowered by increasing the responsivity  or lowering the absolute shot noise. It may be appreciated, therefore, that there remains a need for further advancements and improvements, thus facilitating a micromechanical pixel with improved thermal sensitivity and response time.
Accordingly, what is required is a micromechanical pixel with enhanced responsivity without adversely affecting thermal properties of the pixel.
What is also required is a micromechanical pixel with enhanced thermal response time without adversely affecting the responsivity of the pixel.